The invention relates to a method for the metal-cutting machining of cylindrical shapes, in particular of eccentrically mounted cylindrical shapes on a workpiece rotatable about its longitudinal axis, in particular of crank pins by means of a rotatably drivable tool wherein a finished shape is produced such that it either because of sufficiently good surface finish has the final dimensions or that it is oversize for carrying out a finish machining such as grinding or polishing, as well as to an apparatus for carrying out the method and the cutting insert formed therefor.
According to the known method crankshaft pins are machined by lathing or rotary lathing with the crankshaft eccentrically mounted so as to produce a purely rotary movement about the crankpin axis. With this mounting, machining of the cheeks is also possible but this type of machining has shown itself to be trouble-prone, in particular the chucks for eccentric mounting of the crankshaft are expensive. In addition the machining by lathing with a tool is done such that in a single operation a finished shape is produced that only needs in the region of the pin a finish machining by grinding. With this machining a different tool must be used for each different crank.
According to the method of the prior art, tool carriers are used with several cutting-insert types used on the tool carrier. By means of this tool carrier the cheeks and undercuts to both sides of the shape and the entire length of the pin are finished in one cutting operation. Thus a tool carrier is needed for each finished shape. According to a special goal of the method in a first step the cheeks are machined and in a second machining step the pins are machined to produce the pin diameter and the undercuts.
According to a second known method the crankshaft is clamped and machined with an inside miller that is moved in an orbit about the crank pin. Disadvantages of this machining are large movements and large moved masses so that high-speed machining is not possible.
A milling head for producing a bearing shape on a crank shaft in one step is known from German 3,824,348. The suggested milling head of the disk or collar type for machining crankshafts which have as surfaces to be machined by milling a cylindrical surface extending parallel to the milling-head axis and preferably merging via a rotation-symmetrical groove into a surface nearly perpendicular to the axis, with on the outside or inside cutter periphery of the milling head sets of indexable cutting plates secured in holders, is so constructed that several sets of index-able cutting plates are arranged uniformly around the cutter periphery of the milling head, that each set of indexable cutting plates fully covers the central shape of the surface of the work-piece to be machined and that at least one indexable cutting plate is provided inside one set of indexable cutting plates for each surface to be individually machined, and is so fitted with special indexable cutting plates that all the surfaces can be simultaneously machined.
An indexable cutting plate which has a polygonal flat body with at least one main cutting edge, a cutting edge, and a free face, is known from German 4,400,570.
Also known is so-called high-speed milling, that is a metal-cutting machine with a miller (outside miller) that works
a) with a high cutting speed of more than 160 m/min,
b) with a thin chip thickness in the region of 0.15 mm, preferably in the region of 0.05 mm to 0.1 mm, and
c) with a reduced cutting-arc length.
The term cutting-art length refers to the length of the tool carrier engaged with the corresponding cutting insert relative to its overall circumference.
High-speed milling produces such a good surface quality that pretreating or additional treatments before heat treatment as a separate step can be completely eliminated. In addition this type of milling, although ideal because of its shortened machining time and better surface quality, is not used to date with the machining of cylindrical shapes, in particular crankshafts because with the known tools
a) as a result of the considerable chip thickness and the relatively small engaging cutters at any time because of the very varied force distribution, considerable vibration is produced,
b) the use of indexable cutting plates with negative cutting angle leads to high machining temperatures, and
c) the high machining speed produces such rapid tool wear that tight tolerances along the pin cannot be guaranteed, so that as a result the indexable cutting plates must be changed often which effectively negates the advantages of high-speed milling.
Starting with the state of the art it is an object of the invention to improve the metal-cutting machining of cylindrical shapes, in particular crankshaft pins, so that while keeping or improving the surface quality and while keeping or improving the dimensional accuracy one achieves a reduction in the machining time so that at the same time the overall efficiency is increased.
This object is achieved according to the invention by a method of metal-cutting machining of a cylindrical shape in a workpiece that comprises the steps of:
continuously rotating a workpiece about a workpiece axis;
continuously rotating about a tool axis offset from the workpiece axis a tool having a periphery provided with a plurality of cutting inserts; and
bringing the inserts into radial engagement with the workpiece while orienting at least one part of each cutting insert as it contacts the workpiece relative to at an effective cutting angle between xe2x88x925xc2x0 and +15xc2x0 or a positive axial angle.
The inventive method is a machining system which is particularly suitable for the metal-cutting machining of eccentrically mounted cylindrical shapes, in particular crankshaft pins, with a centrally mounted workpiece. Thus both the difficult-to-finish pins as well as the main journals can be finished on the same apparatus or with serial or parallel arrangement of several apparatuses on the same apparatuses.
The production of the finished shape that has the desired finished dimension or is slightly oversize so that it can be reduced by grinding or polishing by means of tools that are equipped with adjacent cutting inserts or that are simultaneously engaged with the workpiece is called cut distribution. This covers for example the separation of the machining of the cheeks of a crankshaft and the machining of the undercuts and the pins of a crankshaft or the production of half a crankshaft-cheek shape with an undercut and a diameter part with a first tool and the production of a second undercut and the remaining diameter part with a second tool. By such a separation or machining with separate tools it is possible to vary the pin length within certain limits, that is even produce pins of different length with a right and a left tool and/or to correct changes in the pin length as a result of tool wear by adjustment with an NC controller.
The machining is carried out in that each tool is driven with constant or variable high speed so that high-speed machining is used. Preferably the variable speed is changed linearly in steps. As a result the advantages of high-speed milling can be achieved even when machining a crankshaft, namely increasing the surface quality, increasing the dimensional accuracy, and reduction of the machining time. For the use of high-speed milling it was necessary to overcome different disadvantages and problems. Thus the use of the already known cutting inserts in existing tools in known apparatus was not possible as they resulted in excessive forces and too much warming for the tools. With flexible crank shafts there is in some cases a great tendency to vibrate. This can be countered in that at least two millers are used. By such division several advantages are obtained. While the use of a single tool creates axial and radial forces which produce forces which can result in considerable vibration, the division can be set such that this is largely avoided. In addition is becomes possible, in particular with NC control of tool advance, that wear of the tool or of the cutting inserts can be compensated for. In addition the workpiece can be of any size so that the finishing of different crankshafts can be done without difficulty on the same production line.
With the procedure according to the invention with two tools that can be considered a xe2x80x9csplit tool,xe2x80x9d there are various advantages since different shapes can be produced with the same tools and it is possible to make a correction to compensate for wear of the cutting inserts, in particular of indexable cutting plates.
According to the invention the tools are high-speed millers provided with the necessary cutting inserts. According to a special embodiment, the cutting inserts are so constructed and mounted that in a single workpiece advance half of the finished shape of the pin is produced. Preferably a disk miller is used.
According to a particular embodiment of the invention, indexable cutting plates are provided on one tool for machining the cheek plates and on another tool for finishing the crank pins and for creating the undercuts. All the cutting inserts have a positive cutting angle. The cutting-insert setting relative to the tool is set so as to form an effective cutting angle between xe2x88x925xc2x0 and +15xc2x0, preferably between xe2x88x925xc2x0 and +5xc2x0 and/or a positive axial cutting angle (back cutting angle xcex3p) up to 10xc2x0.
According to a further particularly advantageous embodiment of the invention the cutting inserts are mounted on two tool holders such that a first tool cuts the right-hand side of the shape including the adjacent half of the pin and a second tool cuts the left side of the shape including the adjacent other half of the pin.
To correct an overcut a cutting insert specially made according to the invention is used that is described below structurally. An overcut is the overlap region in a workpiece surface being machined which is created on machining by means of two rows of cutting plates or two tools. As a result of preparation precision or, with a split tool, by precise positioning of the machine there is a height offset between two cutting plates or cutting-plate rows. This produces in the overlap region an undesired sharp-edged offset. In order to avoid this the cutting faces of the preferably used cutting inserts in the region of the main cutting edge form at a cutting corner a flat or recess that extends in the region of the two adjacent main cutting corners. The use of such cutting inserts makes it possible to produce extremely gentle transitions in the overlap region that result in an optically and measurably optimum surface, in particular in the pin region of a crank shaft.
The subdivision of the cut, that is the division of the workpiece finishing into two finishing operations, ensures that during the milling operation fewer blades are simultaneously in contact so that the force of the tool and the likelihood of vibrations is reduced.
As a result of the use of a cutting insert with a positive cutting angle the cutting temperature is lowered along with the forces effective on the tool and the likelihood of vibration.
A further advantage of the method according to the invention is that in the case of separation of the cutting inserts, in particular of indexable cutting plates for making right and left halves of the shape, the wear at the cutting inserts can be compensated for by moving the tool axially or radially.
If the complete shape is produced by a single tool, at least one indexable cutting plate has to be set back, that is manually offset, in order to make equal the wear on both sides.
A further feature of the invention is that at least one tool carrier is an orthogonal miller.
The apparatus for carrying out the machining operation according to the invention should have one support per crankshaft bearing which is outfitted for milling operations for movement along at least one feed axis and for orthogonal rotary milling along two feed axes so that the workpiece can be moved in dependence of the rotation angle of the workpiece such that after the metal-cutting machining of the eccentrically arranged crank-shaft pins a cylindrical correction is produced and the above-described correction procedures are carried out for compensating out wear of the indexable cutting plates.
The special procedure is that the tool is moved along one axis in dependence on the angular position of the workpiece such that after the metal-cutting machine a cylindrical shape is produced. As a result of the high cutting speed and the small chip thickness only modest forces are brought to bear on the workpiece which facilitate machining a flexible workpiece. Simultaneously as a result of the modest rate of advance and the high cutting speed it is possible to produce a better surface quality than has been achievable with similar systems.
Since the entire periphery of the tool is large relative to the actual cut-arc length each individual cutting insert is used only briefly as the tool rotates so that it has ample time to cool. In addition it is possible to use a large number of cutting inserts so that each tool can be counted on to have a long service life.
It is particularly advantageous when according to the invention the high-speed milling is combined with orthogonal rotary milling since the advantages of both types of machine can be optimally combined with each other. Preferably the apparatus for carrying out the method has to this end several supports so that the tool is mounted for orthogonal rotary milling on two slides in order that the tool can follow the workpiece.
The subdivision of the tool and the use of at least two milling steps ensures that preferably the shape is produced by two tools which are mounted on two driven tool carriers in the apparatus. The advantages of a split tool concept relative to the earlier setting of the shape as a complete shape by a single tool in one cutting operation are that
1. The reduction in tool width caused by wear can easily be compensated for by moving the tool.
2. As a result of the lightly angled indexable cutting plate (cutting insert) the finished shape has minimal offset errors.
3. The cut width of the crank pin to be made is easily changed, that is according to setup the produced pin length can be changed with the same tool by about 5 mm.
4. Loading of the machine and workpiece is maintained constant by the small unitary cutting width.
An apparatus according to the invention is thus constructed such that the cutting operation for producing the finished shape is done in at least two cutting operations (cut division) so that the tool holders equipped with indexable cutting plates are engaged one after the other and the tool carrier is only equipped with indexable cutting plates that have a positive cutting angle so that the apparatus has at least two tools provided with cutting inserts with each tool being a high-speed miller provided with the necessary cutting inserts. Preferably a disk miller is used as tool which is provided with the corresponding indexable cutting plates that allow high-speed milling. The pins are machined optimally with an orthogonal rotary miller.
For carrying out such a method or for using the apparatus according to the invention it has been shown to be advantageous when a cutting insert is used having a generally perpendicular cutting face that is peripherally bounded by respective main cutting edges, free faces set at a positive angle and each of whose common edges form auxiliary cutting edges, and a positive cutting angle. The cutting face of the insert has in the region of an adjacent main cutting edge forming a cutting corner a flat or recess which extends into the region of both adjacent main cutting edges. The cutting insert has as is known a flat polygonal body and is preferably constructed such that when used on the workpiece a positive cutting angle is produced. The positive cutting angle lowers the machining temperature and in addition the forces exerted on the tool and thus the likelihood of vibration is minimized. In addition the shape is such that the sum of the free angle and the wedge angle is between 85xc2x0 and 95xc2x0, giving an effective cutting angle xcex3f between xe2x88x925xc2x0 and +5xc2x0. If necessary the cutting angle can be as much as 15xc2x0. The back cutting angle xcex3p is set positive and should be at most 10xc2x0. With a tangential mounting position of the cutting body the free face is used as effective cutting face as defined above while oppositely the cutting face is used as effective free face. Similarly the effective free face has the above-described flats or recesses. According to function per cutting insert two or even four flats or recesses are possible.
It has been shown that the width of the cutting body or the width of the main cutting edge being used is preferably some-what larger than half of the length of the pin being machined so that as a result of the use of two succeeding inserts of two cutting bodies it is possible to produce an optimal pin surface.
In particular to optically and measurably optimize the overlap region when using two cutting-insert sets so that a nearly perfectly uniform pin surface is produced, a special cutting insert is suggested wherein for at least one tool a support is provided which allows movement of the tool in two mutually perpendicular directions. This special formation is optimally customized for the application. It is particularly advantageous when the cutting face in the region of the cutting-corner corners has a flat or recess, that is during the manufacture of such a cutting insert a flat is formed in the region of the cutting-body corners which ensures that the cutting corner is set back from the main cutting edge. This leads to a softer transition in the overlap region of the insert region of two such inert bodies so that in trials, in particular of the pin surface of a crankshaft, both a measurably and optically optimal surface is produced.
In order to further increase the service life of such a cutting insert which is in particular loaded heavily in the region of the cutting-body corners, flats or recesses are formed in the region of the cutting corners. Such a cutting plate is particularly stable when used in a so-called split tool for separate cuts. The diameter-producing cutting plate of the first tool thus cuts with its dangerous cutting angle 90o into the solid material of the workpiece. As a result of forming support surfaces on the free faces of the cutting corners an additional stabilization is provided in these regions without dropping the positive chip-shaping geometry and/or the four cutters of the cutting plate.